Analysing the quality of Swiss National Forest Inventory measurements of woody species richness

Abstract

Background: Under ongoing climate and land-use change, biodiversity is continuously decreasing and monitoring biodiversity is becoming increasingly important. National Forest Inventory (NFI) programmes provide valuable timeseries data on biodiversity and thus contribute to assessments of the state and trends in biodiversity, as well as ecosystem functioning. Data quality in this context is of paramount relevance, particularly for ensuring a meaningful interpretation of changes. The Swiss NFI revisits about 8%-10% of its sample plots regularly in repeat surveys to supervise the quality of fieldwork. Methods: We analysed the relevance of observer bias with equivalence tests, examined data quality objectives defined by the Swiss NFI instructors, and calculated the pseudo-turnover (PT) of species composition, that is, the percentage of species not observed by both teams. Three attributes of woody species richness from the latest Swiss NFI cycles (3 and 4) were analysed: occurrence of small tree and shrub species (1) on the sample plot and (2) at the forest edge, and (3) main shrub and trees species in the upper storey. Results: We found equivalent results between regular and repeat surveys for all attributes. Data quality, however, was significantly below expectations in all cases, that is, as much as 20%-30% below the expected data quality limit of 70%-80% (proportion of observations that should not deviate from a predefined threshold). PT values were about 10%-20%, and the PT of two out of three attributes decreased significantly in NFI4. This type of uncertaintytypically caused by a mixture of overlooking and misidentifying speciesshould be considered carefully when interpreting change figures on species richness estimates from NFI data. Conclusions: Our results provide important information on the data quality achieved in Swiss NFIs in terms of the reproducibility of the collected data. The three applied approaches proved to be effective for evaluating the quality of plot-level species richness and composition data in forest inventories and other biodiversity monitoring programmes. As such, they could also be recommended for assessing the quality of biodiversity indices derived from monitoring data.

A century of national forest inventories – informing past, present and future decisions

“…The current importance of information on carbon stock changes and ecosystem services is underlined by four studies based on the Swiss NFI. Traub and Wüest (2020) analyze the quality field data of the woody species composition and provide several approaches for doing so. They find that data quality was as great as 30% less than the expected data quality limit, and the percentage of omitted species was as great as 20% less.…”

Nested plot designs used in forest inventory do not accurately capture tree species richness in Southwestern European forests

“…Species richness in NFIs is frequently estimated as the number of tree species assessed in the plot area corresponding to the "Indicator 4.1. Tree species composition of Forest Europe" (Chirici et al 2012;Forest-Europe 2015;Traub and Wüest 2020). Tree species richness derived from NFIs has been widely used in various applications, such as monitoring forest diversity at different scales (Ghadban et al 2021;Bravo-Oviedo et al 2021) or as a predictor in modelling approaches (Ruiz-Benito et al 2012).…”